Update readme

Author: Frank Schmid

README.md

@@ -6,7 +6,23 @@ The module can be used to convert keys into buffers or other formats to
 enable the direct use of JWK formatted keys with other node modules like
 [njwt](https://www.npmjs.com/package/njwt) and others.
 
+## Usage
+
+_will come_
+
+## Documentation
+
+_will come_
+
+## Supported key types
+
+RSA, EC, oct
+
+All keys but binary (oct) keys can be converted into PEM format for their
+public and private keys.
+
 ## References
 
 [RFC-7517 JSON Web Key](https://tools.ietf.org/html/rfc7517)
+
 [RFC-7518 JSON Web Algorithms](https://tools.ietf.org/html/rfc7518)